# p2p chat app with libp2p [support peer discovery using mdns] This program demonstrates a simple p2p chat application. You will learn how to discover a peer in the network (using mdns), connect to it and open a chat stream. This example is heavily influenced by (and shamelessly copied from) `chat-with-rendezvous` example ## How to build this example? ``` go get -v -d ./... go build ``` ## Usage Use two different terminal windows to run ``` ./chat-with-mdns -port 6666 ./chat-with-mdns -port 6668 ``` ## So how does it work? 1. **Configure a p2p host** ```go ctx := context.Background() // libp2p.New constructs a new libp2p Host. // Other options can be added here. host, err := libp2p.New() ``` [libp2p.New](https://godoc.org/github.com/libp2p/go-libp2p#New) is the constructor for libp2p node. It creates a host with given configuration. 2. **Set a default handler function for incoming connections.** This function is called on the local peer when a remote peer initiate a connection and starts a stream with the local peer. ```go // Set a function as stream handler. host.SetStreamHandler("/chat/1.1.0", handleStream) ``` ```handleStream``` is executed for each new stream incoming to the local peer. ```stream``` is used to exchange data between local and remote peer. This example uses non blocking functions for reading and writing from this stream. ```go func handleStream(stream net.Stream) { // Create a buffer stream for non blocking read and write. rw := bufio.NewReadWriter(bufio.NewReader(stream), bufio.NewWriter(stream)) go readData(rw) go writeData(rw) // 'stream' will stay open until you close it (or the other side closes it). } ``` 3. **Find peers nearby using mdns** Start [mdns discovery](https://godoc.org/github.com/libp2p/go-libp2p/p2p/discovery#NewMdnsService) service in host. ```go ser, err := discovery.NewMdnsService(peerhost, rendezvous) ``` register [Notifee interface](https://godoc.org/github.com/libp2p/go-libp2p/p2p/discovery#Notifee) with service so that we get notified about peer discovery ```go n := &discoveryNotifee{} ser.RegisterNotifee(n) ``` 4. **Open streams to peers found.** Finally we open stream to the peers we found, as we find them ```go peer := <-peerChan // will block until we discover a peer fmt.Println("Found peer:", peer, ", connecting") if err := host.Connect(ctx, peer); err != nil { fmt.Println("Connection failed:", err) } // open a stream, this stream will be handled by handleStream other end stream, err := host.NewStream(ctx, peer.ID, protocol.ID(cfg.ProtocolID)) if err != nil { fmt.Println("Stream open failed", err) } else { rw := bufio.NewReadWriter(bufio.NewReader(stream), bufio.NewWriter(stream)) go writeData(rw) go readData(rw) fmt.Println("Connected to:", peer) } ``` ## Authors 1. Bineesh Lazar